1. Field of the Invention
The invention relates to an engine control system, and in particular, relates to a control system that calculates engine torque in order to control an engine.
2. Description of the Related Art
Generally, as described in Japanese Patent Application Publication No. 8-291761 (JP-A-8-291761), an engine control apparatus has come into practical use in which it is determined whether a combustion state in an engine is good based on information on an angular velocity of a crankshaft, and the determination result is fed back to the engine control. The determination as to whether the combustion state is good may be made based on the angular velocity information, for example, in the manner as described below. First, the engine torque is calculated based on the angular velocity of the crankshaft. In a simple calculation method, the engine torque is calculated by multiplying an angular acceleration of the crankshaft by a moment of inertia of the crankshaft. Thus, it is possible to determine whether the combustion state in the engine is good based on the amount and pattern of fluctuation of the engine torque.
A hybrid vehicle, which includes two types of power sources, an engine and an electric motor, has come into practical use. The hybrid vehicle may be configured so that the crankshaft is operatively connected to a rotor of the electric motor. In this case, the following phenomenon may occur. Because the engine is operated by repeating intermittent combustion therein, rotation of the crankshaft fluctuates in synchronization with the combustion cycle of the engine. If the rotor of the electric motor is directly connected to the crankshaft, the rotor rotates at a non-constant speed. This may affect operation and control of the electric motor. In consideration of this, in the hybrid vehicle thus configured, a low-rigid torsional damper is interposed between the crankshaft and the rotor of the electric motor. Thus, the torsional damper functions to avoid direct transmissions of the rotational fluctuation of the crankshaft to the electric motor by damping the rotational fluctuation of the crankshaft.
However, in the engine in which the low-rigid torsional damper is connected to the crankshaft, it is not possible to calculate the engine torque with sufficient accuracy based on only the information on the angular velocity of the crankshaft.
More specifically, if the torsional damper is connected to the crankshaft, torque produced by the engine is partially absorbed by torsion of the torsional damper. A torsional damper provided in a typical non-hybrid vehicle is sufficiently rigid and is regarded as substantially a rigid body. Therefore, it is possible to calculate the engine torque with sufficient accuracy even when the amount of torque absorbed by torsion of the torsional damper is not taken into account. However, because a relatively low rigid torsional damper is employed in the hybrid vehicle as described above, the amount of torque absorbed by torsion of the torsional damper becomes large to the extent that cannot be ignored. Moreover, when the torsional damper rotates, the torsional damper is repeatedly twisted and untwisted in accordance with a change of the angular velocity of the crankshaft caused by a change of an engine load, and therefore, the amount of torque absorbed by the torsion of the torsional damper significantly fluctuates as time passes. Thus, in the engine in which the low-rigid torsional damper is connected to the crankshaft, it is not possible to calculate the engine torque with sufficient accuracy based on only the information on the angular velocity of the crankshaft.